1. Field of the Invention
The present invention relates to a separator having an inorganic protective film and a lithium battery using the same, and more particularly, to a separator having an inorganic protective film with suppressed self discharge and reduced internal short, and a lithium battery using the same.
2. Description of the Related Art
With development of lightweight, high-function portable electronic devices such as video cameras, cellular phones, notebook PCs and the like, batteries used as driving sources thereof have been extensively studied. In particular, rechargeable lithium batteries have attracted much attention and development thereof are briskly under way because they have approximately three times a higher energy density than conventional batteries such as Ni—Cd batteries, Ni—H batteries or Ni—Zn batteries and are capable of charging rapidly.
An electrochemical cell of a battery is basically composed of a cathode, an anode and a separator interposed therebetween. During discharge, oxidation takes place at the anode, and reduction takes place at the cathode. Lithium ions generated during oxidation and reduction migrate through an electrolyte which is impregnated in pores of the separator. The cathode and the anode are not electrically contacted to each other by the separator, so that electrons cannot move directly between the anode and the cathode.
The porous separator used in a cell should be chemically stable in an electrolytic solution, and useful examples thereof include polyolefins such as polyethylene or polypropylene, glass fiber filter paper or ceramic materials. In particular, like in a cylindrical battery in which a separator is wound circularly, the separator must have sufficient flexibility. Also, along with the high energy density trend, a separator must have a mechanical strength even with a reduced thickness.
In the case where a cathode active material is easily dissolved in an electrolytic solution to thereby be able to move easily, a separator must be equipped with a function of shutting down a cathode active material. In a lithium sulfur battery, in particular, sulfur used as a cathode active material forms lithium polysulfide which is easily dissolved in an electrolytic solution during charge and discharge. The lithium polysulfide moves to an anode and reacts with metallic lithium to form side reactant materials, lowering a storage characteristic of a cell. In other words, a severe self discharge problem occurs.
In such a case, a reduction in capacity due to self discharge can be noticeably suppressed by shutting down movement of lithium polysulfide. To this end, a polymer such as poly(ethylene oxide), poly(propylene oxide), poly(vinylidenefluoride), poly(tetrafluoroethane) or poly(acrylonitrile) is coated on the surface of a separator, thereby retarding movement of a cathode active material to some extent. Shutting down of the active material is restricted due to swelling of the polymer used. To overcome this problem, a polymer that does not causing swelling may be used. However, in such a case, ion permeation is also inhibited, making it difficult to realize battery performance.